This invention relates generally to the application of ultraviolet-visible light measurements for the determination of colloidal substances in a liquid sample. More particularly, the invention relates to the application of light attenuance and/or light scattering measurements for determining and controlling a property of colloidal substances that undergo at least one of a temperature dependent phase transition and a temperature dependent adsorption-desorption process.
Papermaker""s demands for high speed and efficiency, flexible manufacturing, stringent quality standards, and environmental compatibility coupled with new developments in on-line process control are driving the development of new sensor technology for the paper machine wet-end. The need for better systems for providing wet-end chemistry control is emphasized by recent reports stating that only 10% of the world""s 150 newsprint paper machines operate at an efficiency above 88%, and that more than 60% operate in the low efficiency range below 82.5%, see for example Mardon, J., Chinn, G. P., O""Blenes, G., Robertson, G., Tkacz, A. Pulp and Paper Canada, Vol. 99 No. 5 pp. 43-46. (1998). William E. Scott addressed some of the problems that are related to wet-end chemistry control in Principles of Wet End Chemistry. Tappi Press, Atlanta, 1996. p. 3. xe2x80x9cDeposits and scale usually arise from out-of-control wet end chemistry. Typical examples include chemical additive overdosing, charge imbalances, chemical incompatibility and the shifting of chemical equilibria. All of these phenomena can lead to the formation of precipitates or colloidal aggregates that produce deposits and scale. While there are numerous approaches to treating the symptoms of deposits the best approach is to determine what is out of control and fix it.xe2x80x9d
Pitch is a generic term for the colloidal components that are suspended in the pulping process waters or water of the paper machine wet-end, wherein the composition of individual pitch particles may vary from relatively pure mixtures of fresh resin and fatty acids to heterogeneous agglomerations of wood extractives, wood-derived lignin and hemicellulose, salt, cationic polymer and filler particle. The common use of the term pitch often blurs the distinction between extractives, pitch, white pitch, and stickies as defined in the Dictionary of Paper 5th Edition, Tappi Press, 1996, Atlanta Ga. The degree to which temperature will alter the equilibria between colloidal pitch and dissolved substances is a complicated function of the solution conditions and the composition of the pitch particle itself. In the field of pulp and paper manufacture the maintenance of a level of stability and/or the removal of colloidal pitch is an important objective in the wet-end chemistry control programs. Deposition leading to poor paper machine efficiency is a costly problem, which is addressed through numerous strategies including pulp processing optimization and/or addition of chemical agents.
Nearly every pulp and paper mill has a strategy for controlling pitch to prevent its deposition on pulping, bleaching and papermaking machinery and to reduce pitch build-up in white water systems. Pitch control strategies include: stabilization with dispersants; coagulation and fixation with cationic polymers; and, adsorption and removal with mineral additives such as talc or bentonite clay, as described by Garver Jr., T. M. and Yuan, H. Measuring the response of pitch control strategies. in PAPTAC 87th Annual Meeting. 2001. Montreal, the contents of which are incorporated by reference herein. A strategy utilizing dispersants, such as for example non-ionic surfactants, is usually employed in open systems where the dispersed colloids can go to drain, while a strategy of removal with cationic polymers or talc is typically used in closed systems where the incoming pitch must be removed with the product. In addition, there is a plurality of pulp processing variables that may be adjusted in an attempt to reduce pitch accumulation and deposition. Ultimately the single most important reason for controlling pitch is the cost of lost production time related to cleaning pitch deposition on equipment. Other factors related to product quality include such issues as increased dirt and speck counts from agglomerated pitch or sloughed off deposits and loss of paper strength related to the interference of resinous substances with interfiber bonding and surface tension that is important for wet strength.
A method for controlling pitch using micro-particle bentonite addition with cationic polymer flocculation is disclosed in U.S. Pat. No. 5,676,796, issued to Cutts on Oct. 14, 1997. Another combination using kaolin as inorganic colloid and poly(diallyldimethyl-ammonium chloride) cationic polymer is disclosed by Lamar; Pratt; Weber and Roeder in U.S. Pat. No. 4,964,955, which issued Oct. 23, 1990. Alternatively, Dreisbach and Barton disclose a method of preventing pitch deposits by the addition of a nonionic polymeric dispersing agent in U.S. Pat. No. 5,266,166, issued Nov. 30, 1993. A physical process for reducing wood resin pitch from wood process water employing a centrifuge is disclosed by Allen and Lapointe in U.S. Pat. No. 5,468,396, issued Nov. 21, 1995. Of course, any method for controlling pitch by the addition of chemical agents requires rapid analysis of the process water to avoid accidental overdosing or underdosing of the chemical agents, which would produce undesirable results or which would increase unnecessarily the overall cost of the process control program.
A plurality of instruments for relating the intensity and/or angular dependence of scattered or absorbed light to the total concentration or size distribution of colloids are known in the prior art. For example, instruments for characterizing the amount of colloidal particles on the basis of light scattering measurements (nepholometry) and light attenuation measurements (turbidimetry) are commercially available as laboratory, hand-held and on-line instruments. On-line turbidimeters measure the intensity of light that is detected in-line and at an angle to a source of incident light. The turbidimeter relates a ratio of the detected intensities of light to a turbidity value in Jackson or NTU units. U.S. Pat. No. 4,999,514, issued Mar. 12, 1991 in the name of Silveston, discloses a method for controlling the intensity of the light source to provide a turbidimeter that operates over a broad range of particle concentrations. Kubisiak and Wilson in U.S. Pat. No. 5,331,177 describe an analog to digital turbitimeter apparatus that provides a measure of the change in turbidity over time. Other, more sophisticated methods, involving the analysis of the time and spatial dependence of light attenuation and scattering may provide information on particle size distributions, as taught by Strickland et al. in U.S. Pat. No. 5,576,827 and the patents referenced therein. Instrumentation specifically designed for measuring particle and fiber size distributions in low consistency ( less than 1%) pulp suspensions by analysis of the time and spatial variation of scattered or absorbed light includes the BTG-Spectris RET-5300 or the Metso Automation RM200 Retention Monitoring System. These instruments employ methods taught by Lundqvist; Pettersson and Fladda in U.S. Pat. No. 4,318,180.
Unfortunately, the prior art instruments are other than capable of differentiating colloidal wood-derived pitch particles from similarly sized colloidal clay particles and hence the measured concentration represents the total concentration of all colloidal species in suspension. As such, methods to measure the amount of a specific colloidal component in a mixture or to rapidly evaluate the temperature stability of a colloidal suspension are not readily available. Additionally, systems for obtaining on-line measurements of colloid tackiness or the propensity for a colloid to agglomerate or to deposit onto a surface currently are not available. Further additionally, no measurement systems exist currently that provides a measure of the capacity of a colloid to adsorb and/or desorb other dissolved or colloidal substances.
Ideally, on-line measurements would assist with selection and/or design, optimization and control of chemical programs to reduce pitch deposition. These measurements would evaluate deposition rate and propensity toward deposition as the most relevant feedback indicator for the control of pitch management programs. In general, the currently available methods for evaluating pitch concentration, deposition, deposition tendency and tackiness are time consuming and provide insufficient reproducibility and insufficient information to be generally useful for the control of pitch in paper mills. Methods to evaluate and systematically improve pitch control chemical formulations and on-line methods for quantifying pitch are practically nonexistent. Laboratory methods, however, are known and include quantification of pitch concentration by colloidal pitch counts using microscopy or a laser particle counter. Allen in Trans. Tech Sect disclosed the accepted laboratory method of pitch analysis using microscopy. CPPA 3(2):32 (1977). This procedure is time consuming, as it has not yet been successfully automated by computerized image analysis techniques. An instrumental method employing a laser beam to count particles flowing through a capillary has been described by Eisenlauer; Horn; Ditter; Eipel in U.S. Pat. No. 4,752,131. The laser method, known as a pitch counter, requires expensive and specialized instrumentation that is not easily adapted to analysis in an industrial setting.
The turbidity of a suspension is also widely used as a measure of the concentration of suspended pitch particles, and there is some hope of achieving on-line turbidity measurements of colloids removed from a fiber suspension. Turbidity measures a concentration of colloids, however, the propensity for colloids to deposit is governed not only by the concentration but also by the forces between surfaces and the colloid particles. To the best of our knowledge, a variable temperature turbiditimeter currently is not commercially available.
Several technically simple tests for gravimetrically measuring deposition have also been described in the prior art. Other more sophisticated techniques, such as atomic force spectroscopy and jet impingment, may be used to measure deposition or intersurface forces, but methods to directly measure and characterize deposition or pitch tackiness are time consuming and inappropriate for use in mill evaluation. It is for this reason that the factors influencing deposition, such as for example concentration and electrostatic forces, are typically measured. The best available laboratory or on-line instrumentation for characterizing the interaction energies between two particles or between a particle and a surface are instruments for measuring the electrostatic potential at the surface of colloid particles. Zeta or streaming potential or charge measurements are different ways to characterize electrostatic charge and they may be used to monitor and control chemical and physical methods for controlling pitch. Although the surface or total charges are important measures of the colloidal stability, these measurements do not distinguish colloidal pitch from other, less problematic colloids such as added clay. In addition, the measurement of the electrostatic forces for suppression of deposition by a pitch control agent ignores the importance of competing attractive London dispersion forces. Although some efforts have been made to control pitch in dependence upon on-line and off-line turbidity or charge measurements, typically chemical methods for controlling pitch are performed absent real-time feed back control. Despite the high cost of chemical treatment programs and the potential downtime caused by overdosing or underdosing, chemical methods for controlling pitch are often invariant over time, and substantial swings in wet-end chemistry may result.
In view of the need for better instrumentation for pitch characterization, it would be advantageous to provide an instrumental method that characterizes pitch concentration, composition and stability by analysis and interpretation of the wavelength dependence of light attenuation by mixtures of pitch particles. This method may be used to evaluate the effectiveness of chemical treatments for pitch control. The method and the apparatus in accordance with the invention provides for on-line measurements of colloidal substances in a liquid sample. This invention is particularly useful for determining or estimating the amount of colloidal substances in pulp or paper mill process water or effluents. The method and the apparatus in accordance with the invention are able to empirically identify and measure a property related to the size, composition, and concentration of a colloidal mixture. The measurements thus obtained according to the instant invention provide a means to empirically identify and measure a property related to the particle size, composition, and concentration of a colloidal mixture.
It is an object of the instant invention to provide a method and apparatus for rapidly evaluating the response of a characteristic of a colloidal mixture to an addition of an additive thereto.
It is a further object of the instant invention to provide a method and apparatus for providing a feedback signal to a process controller in dependence upon a measured characteristic of the process.
It is an object of the instant invention to provide a method of measuring the propensity of colloids to deposit, to adsorb substances from solution or to aggregate.
In accordance with an aspect of the invention there is provided a method for controlling a characteristic of a colloidal mixture comprising the steps of:
providing a colloidal mixture for analysis by thermal difference spectroscopy; determining a value indicative of the characteristic of the colloidal mixture by the steps of:
irradiating at least a first portion of the colloidal mixture with light in an ultraviolet-visible region at a first temperature and obtaining a first measurement of a first wavelength within the ultraviolet-visible region, said first measurement for obtaining a measure of one of an absorption, emission and scattering of the first wavelength when said colloidal mixture is irradiated with the light,
waiting for the temperature of the colloidal mixture to change,
irradiating at least a second portion of the colloidal mixture with light in an ultraviolet-visible region at a second different temperature and obtaining a second measurement of the first wavelength within the ultraviolet-visible region, said second measurement for obtaining a measure of one of an absorption, emission and scattering of the first wavelength when said colloidal mixture is irradiated with the light, and
determining the value indicative of the characteristic of the colloidal mixture from a relationship including the first measurement and the second measurement;
determining, in dependence upon the determined value, an adjustment to at least a variable of a control process for affecting at least a characteristic of the colloidal mixture;
providing a feedback signal in dependence upon the determined adjustment to an automated controller of the control process; and
adjusting automatically the variable of the control process in dependence upon the provided feedback signal.
In accordance with the aspect of the invention there is further provided a method for controlling a process parameter of a process involving a colloidal mixture comprising the steps of:
providing at least a first portion of the colloidal mixture for an optical measurement by the steps of:
providing a portion of the colloidal mixture to an in-line centrifuge unit to separate particulate matter therefrom, to obtain an approximately fiber-free sample, and
providing the approximately fiber-free sample as the at least a first portion;
determining a value indicative of a characteristic of the colloidal mixture by the steps of:
irradiating the at least a first portion of the colloidal mixture with light in an ultraviolet-visible region at a first temperature and obtaining a first measurement of a first wavelength within the ultraviolet-visible region, said first measurement for obtaining a measure of one of an absorption, emission and scattering of the first wavelength when said colloidal mixture is irradiated with the light, and
determining the value indicative of the characteristic of the colloidal mixture from a relationship including the first measurement;
determining, in dependence upon the determined value, an adjustment to the process parameter of the process involving the colloidal mixture;
providing to an automated controller of the process involving the colloidal mixture a feedback signal in dependence upon the determined adjustment; and
adjusting automatically the process parameter of the process in dependence upon the provided feedback signal.
In accordance with the aspect of the present invention there is still further provided a method for controlling a characteristic of a colloidal mixture comprising the steps of:
providing a colloidal mixture for analysis by thermal difference spectroscopy; determining a value indicative of the characteristic of the colloidal mixture by the steps of:
irradiating at least a first portion of the colloidal mixture with light in an ultraviolet-visible region at a first temperature and obtaining at least a first measurement of a first and a second wavelength within the ultraviolet-visible region, said first measurement for obtaining one of an absorption, emission and scattering of the first wavelength when said colloidal mixture is irradiated with the light,
waiting for the temperature of the colloidal mixture to change,
irradiating at least a second portion of the colloidal mixture with light in an ultraviolet-visible region at a second different temperature and obtaining at least a second measurement of the first and the second wavelength within the ultraviolet-visible region, said second measurement for obtaining one of an absorption, emission and scattering of the second wavelength when said colloidal mixture is irradiated with the light, and
determining the value indicative of the characteristic of the colloidal mixture from a relationship including a ratio of the at least first and second measurement;
determining, in dependence upon the determined value, an adjustment to at least a variable of a control process for affecting at least a characteristic of the colloidal mixture;
providing a feedback signal in dependence upon the determined adjustment to an automated controller of the control process; and
adjusting automatically the variable of the control process in dependence upon the provided feedback signal.
In accordance with a second aspect of the invention there is provided an on-line optical sensor apparatus for controlling a process parameter of a process involving a colloidal mixture comprising:
an in-line centrifuge unit for separating fiber from the colloidal mixture to obtain an approximately fiber-free liquid sample;
a detector for obtaining a first measurement and a second measurement of light in an ultraviolet-visible region, said first measurement for obtaining a measure of one of an absorption, emission and scattering of at least a first wavelength of the light at a first temperature when the approximately fiber-free liquid sample of the colloidal mixture is irradiated with the light, and said second measurement for obtaining a measure of one of an absorption, emission and scattering of the first wavelength of the light at a second different temperature when the approximately fiber-free liquid sample of the colloidal mixture is irradiated with the light; and
a suitably programmed processor in electrical communication with an automated process controller for providing thereto a signal indicative of an adjustment to the process parameter, wherein said adjustment is determined in dependence upon a characteristic of the colloidal mixture, and wherein said characteristic of the colloidal mixture is determined from a relationship including the first and second measurement.